The challenges of diversion during the stimulations of carbonate formations have been widely studied for decades. It has been shown that viscoelastic surfactants (VES) are among the effective diverting solutions which can be used during the stimulation of carbonate formations. These chemicals benefit from the reaction products between the acid and the calcite to increase the viscosity of the acid, therefore reducing, temporarily, the permeability of the first invaded rock layer, which holds the highest permeability. This temporary blockage steers the remaining acid to the layers with lower permeability that need treatment. This is usually called self‐diverting acidizing treatment. However, the same approach may not necessarily be applied for sandstone formations that are not calcite rich. In such cases, combining acid stages with batches of diverting agents is still feasible. For this approach, VES is also a suitable, non‐damaging solution that is often overlooked. In this study, the application of VES as a diverting agent was investigated during the chemical treatment operations in sandstone formations. Several dual core‐flooding experiments were performed at high pressure and high temperature (HPHT) conditions using sandstone core samples with high permeability contrast.

Matrix stimulation is commonly utilized to increase well productivity and it is a process in which fluid selection plays a key role in treatment success. However, in offshore fields, the acid formulation must not only be effective in damage removal but it also has to meet stricter HSE regulations while safeguarding expensive and complex installations from corrosion impact. In high temperature environments or where sensitive metallurgy is deployed, higher doses of corrosion inhibitors are required as well as additional additives in order to avoid unwanted reactions, further complicating the handling and HSE aspects of such acids. The environmentally friendly chelate, glutamic acid N,N-diacetic acid GLDA, has been examined as an alternative for acidizing and descaling treatments, demonstrating good field performance in terms of productivity and injectivity increase. All this achieved while providing a safe and convenient system for handling, due its low toxicity, fewer required additives and biodegradability. Numerous laboratory tests have measured and confirmed considerably less corrosion risk, across a wide range of conditions, when compared to conventional formulations based on both HCl and organic acids. Within this paper, the field performance of the GLDA system will be evaluated under even more challenging conditions, endured during the acidizing of an offshore well in the North Sea. The wellwork programme for the low rate gas producer consisted of performing two new perforation runs, followed by the injection of the GLDA treatment, which was then bullheaded into the formation with a nitrogen assist. The treatment formulation consisted of GLDA diluted in fresh water with trace amounts of surfactant and mutual solvent to aid in the flow-back of the spent acid. Due to an unexpected power failure on the platform, the treatment remained stagnant in the tubing for some 28 hrs at 300°F. As this was the first GLDA treatment in this field, this situation appropriately raised potential well integrity concerns; as would most certainly have been the case with a conventional HCl acid package. However, once the operation was restarted the acid was successfully bullheaded into the formation and no issues resulted with the low carbon and CRA based metallurgy of the completion even after this extensive unplanned exposure. Additionally the treatment resulted in a significant productivity increase. These operations and results demonstrated not only the gentle nature of GLDA for integrity considerations; but also an effective cleanout of perforations and near wellbore area as required from a replacement system. The success of the treatment proved the intrinsic value and reduced risk that can be accessed by use of such systems.

Economical production from unconventional reservoirs including tight dolomite require some forms of stimulation techniques to increase the effective contact areas between wellbore and formation. However, productivity improvement of these formations with conventional techniques (e.g. acid stimulation) is very limited and mostly unfeasible. In this paper, an efficient chemical treatment is proposed to stimulate tight dolomite formation through wormholing mechanism and scale-based damage removal. The formation damage in tight reservoirs are much more severe due to the smaller pore/throat size. Among them, the scale-based permeability impairment or phase trapping can cause significant production lost. In this study, the proposed treatment fluid is used to remove the scale-based formation damage, mostly caused by drilling mud. To this aim, the damage removal efficiencies of dolomite cores, artificially damaged by scale precipitation, were investigated after HPHT coreflood treatment. In addition, the performance of the treatment fluid was evaluated as a mean to bypass the damaged zones around hydraulic fractures (caused by liquid phase trapping or significant net stress). To evaluate this, a series of coreflooding experiments were also performed on untreated tight dolomite cores and the feasibility of the wormholing mechanism was studied. The permeabilities of tight dolomite rocks were measured before and after the treatment. To visualize the wormhole propagation inside the cores, computed CT scanning were performed. The rock-fluid interaction was also investigated by analyzing the effluent samples by ICP. The main mechanism of this treatment technique is pore body/pore throat enlargement by slow rock dissolution. From the pore scale analysis, it is found that even at lower concentrations, the active ingredient reacts with rock minerals. A damaged dolomite core was also treated, and the results showed that the removal of Barite-based scale can be achieved even in the presence of native calcite or dolomite minerals. Also, it is found that wormholing can be only achieved at certain concentrations (>10 w%). It also depends on the injection rate and other field conditions such as temperature. Even at low concentration, the rock permeability of the damaged dolomite core can be increased by a factor of 35 (Kf/Ki=35). Finally, dolomite reservoir cores (25-30 μD) were treated at low injection rates (0.08-0.1 ml/min) imposed from the well injectivity condition. It was shown that despite an order of magnitude lower injection rate (compared to those in conventional acidizing) still an optimum injection rate is needed to extend the wormhole across the core. It is also verified that the active ingredient can be used in alcohol-based solutions for special applications such as tight gas and gas condensate reservoirs. The corrosion rate is far below the accepted corrosion level of 0.05 lb/ft2 and it is fully compatible with other additives and high salinity brines. The proposed treatment method is cost effective and experimentally proven to be efficient and long-lasting. Such treatment is recommended to tackle the low productivity of unconventional tight reservoirs. This treatment can be even applied to remove the additional formation damages usually caused during conventional stimulations such as hydraulic fracturing to boost the production.

Stimulation systems have improved over past decades, yet challenges prevail in corrosion, unwanted precipitation and handling hazardous chemicals. The role of chelating agents in coping with such concerns, is undeniably positive: their limited corrosivity, effective metal control and outstanding HSE profile, make them effective acidizing alternatives. Particularly when seeking delayed reaction at high temperature or removing insoluble material like Barite, chelating agents like GLDA and DTPA respectively have been reported effective both at laboratory and field scale. Formulations based on abovementioned chelating agents were evaluated experimentally to assess potential stimulation of Kazakhstan formations. Core-plug samples used in this evaluation are predominantly carbonate rock originating from different wells. The coreflooding experiments were performed at HPHT conditions to assess performance of treatment fluids to a) create new flow-channels (wormholes) thus improving rock permeability, and b) remove BaSO4-based solids suspected to be affecting productivity in the field. In this work, five reservoir core plugs were stimulated by GLDA based formulation to assess wormholing mechanism, while two core-plugs were treated by DTPA based fluid to study the impact of matrix cleaning. The matrix cleaning properties of DTPA based fluid were investigated on the damaged core plugs which were artificially damaged by in-situ precipitation of BaSO4 scale. The coreflood study included injection of the preflush, the treatment fluid and the post-flush system at reservoir temperature of 270 °F and low injection rates to accommodate the low permeability of the formation. It was shown that GLDA based fluid can effectively stimulate the reservoir core samples. The effective mechanism was observed to be wormholing thus increasing rock permeability by over a thousand times. No signs of face dissolution were observed despite slow injection rate at such high temperature; something that was not possible when a fast reacting acid (i.e. HCl) was used under the same conditions. In addition, it was shown that the DTPA based fluid can efficiently improve the rock permeability through matrix cleaning by both Barium and Calcium chelation. In the first treatment test by this fluid system, around 45% of the damaged permeability was recovered. While in the second test, not only BaSO4 scale was dissolved but also the CaCO3 minerals were partly dissolved and the core permeability was significantly increased (Kf/Ki >200). Experimental results bring promising prognosis for field implementation despite expected low injectivity at high downhole temperature. GLDA treatments avoid premature acid spending and face dissolution - common outcomes of HCl- which translate into deeper extent of stimulation. Additionally, in barite damaged wells, DTPA treatment represents an attractive solution for damage reduction and by-passing. Finally, intrinsic properties of chelating agents reduce asset integrity risks, improve operation HSE and simplify flow-back handling.

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